1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images to be used in electrophotographic apparatuses that utilize an electrophotographic process, such as copying machines, printers, facsimiles, and the like, and a manufacturing method therefor; a developer for electrostatic latent images using the toner for developing electrostatic latent images; and an image forming method utilizing the toner for developing electrostatic latent images. In addition, the present invention also relates to a method for manufacturing a dispersion of resin particles to be utilized as a constituent material for the toner for developing electrostatic latent images.
2. Description of the Related Art
Methods for visualizing image information through an electrostatic latent image, such as the electrophotographic method, or the like, are widely used in various fields at present. With the electrophotographic method, an electrostatic latent image on the surface of a photosensitive material is developed through a charging step, an exposure process, and the like, and the electrostatic latent image is visualized through a transfer step, a fixing step, and the like.
A number of methods are known as electrophotographic methods. In general, a latent image is formed electrically by one of various means on the surface of a photorecepter (latent image holding material) which utilizes a photoconductive substance. The formed latent image is developed using a toner, and thus a toner image is formed. Thereafter, the toner image on the surface of the photorecepter is transferred onto the surface of a transfer material such as paper or the like via or not via an intermediate transfer material. The transferred image is subjected to a plurality of fixing processes such as heating, pressurizing, heat-pressurizing, and the like, such that a fixed image is formed. Toner which remains on the surface of the photorecepter is cleaned by various methods as necessary and is again utilized for development of a toner image, as required.
As a fixing technique for fixing a transferred image which has been transferred onto the surface of a transfer material, a heat roll fixing method is generally known. In this method, a transfer material, onto which a toner image has been transferred, is inserted and fixed between a pair of rolls which is formed by a heat roll and a pressure roll.
On the other hand, with demand for saving the power required for image formation having increased in recent years, the technological development of the so-called low-temperature fixing toner which is intended to provide electric power saving in the fixing process, which is one of the processes consuming the highest amount of energy in the electrophotographic process, has been actively promoted.
In this case, lowering the fixing temperature for the toner generally means that the glass transition point for the toner is lowered simultaneously, which renders compatibility between the lowered fixing temperature and the desired toner shelflife, and the desired preservability (offset resistance) of the final output image obtained, difficult to achieve. Therefore, in order to render the lowered temperature fixing and the desired toner shelflife compatible, it is necessary to provide the toner with so-called sharp melting characteristics, which abruptly lowers the viscosity of the toner in the vicinity of the fixing temperature, with the glass transition point for the toner being maintained at a high temperature.
As a promising approach to this technological challenge, a method which uses a crystalline resin having sharp melting characteristics as the binder resin has been proposed.
However, it is difficult to obtain crystalline resin particles for use as toner particles, and even if the kneading pulverizing method is used in order to obtain such particles, the pulverization is difficult, the yield being low, which has presented a problem of low practicality from the viewpoint of manufacturability (see, for example, Japanese Patent Publication No. 56-13943 and Japanese Patent Publication No. 62-39428). In other words, as regards improving both the low temperature fixability and the offset resistance, the fusion kneading pulverizing method makes it difficult to use the crystalline resin which is effective in terms of low temperature fixability and offset resistance. Further, because pulverization is conducted made, it has been difficult to control the shape of the toner particles, particularly to sphericalize the toner particles, and to make the diameter of the toner particles smaller and the particle size distribution narrower for the purpose of enhancing the image quality.
As means for solving the above-mentioned problems, a technique which uses a crystalline resin and a non-crystalline resin in combination rather than using the crystalline resin alone as the binder resin has been reported. This is because the existence of the non-crystalline resin moiety facilitates the pulverization. Examples of this method include a method which uses a crystalline resin and a non-crystalline resin in combination (see Japanese Patent Publication Laid-Open (JP-A) No. 2-79860) and a method which uses a resin in which a crystalline resin is chemically bonded to a non-crystalline resin (see JP-A No. 1-163756). However, with the methods as disclosed in these literatures, the percentage of the non-crystalline resin is high, and thus the fusion of the entire toner is governed by the softening temperature of the non-crystalline resin, thus the effect of the low-temperature fixability has been low.
In addition, the dissolution suspension method and other wet type manufacturing methods which manufacture toner particles by a chemical approach have been examined (see JP-A No. 9-15902). When a wet type manufacturing method, such as the dissolution suspension method, or the like, is used, the toner particles, which are difficult to knead and pulverize, are dissolved in an organic solvent to be suspension-dispersed into an aqueous medium and can, thereby be easily manufactured. The shape of the toner particles can be controlled, and thus sphericalized toner particles can be manufactured with ease. The particle size distribution of the toner particles can also be controlled. However, with the dissolution suspension method using a crystalline polymer, it is difficult to uniformly disperse small-amount components, such as the colorant, and the like, into the toner. In addition, there are environmental problems related to using an organic solvent. In addition, the volume resistivity of a crystalline polymer is low, compared to that of a non-crystalline polymer in the molecular weight region which has been conventionally used, thus at present it is difficult to assure the charging characteristic required for electrophotography using a crystalline polymer alone.
In order to alleviate this problem, the emulsion polymerization aggregation method, which, in the solution with which a crystalline resin is dissolved in a radical polymerizable monomer for formation of an amorphous polymer, aggregates and fuses emulsified or suspended resin particles comprising these components to provide toner particles, has been proposed (see, for example, JP-A Nos. 2001-42564 and 2001-42568).
However, with these methods, the percentage of the crystalline polymer comprised in the toner is low, and thus they are not sufficient for obtaining the characteristics of the crystalline polymer. With these methods, characteristics such as low temperature fixability, image preservability, and the like, originating from the sharp melting characteristics inherent to a crystalline resin, cannot be effectively drawn out, and thus sufficient low-temperature fixing characteristics cannot be achieved. Further, for cases as described above, JP-A Nos. 2001-42564 and 2001-42568 indicate that it is important to form a micro-domain structure (the sea-island structure) in the final toner, and that the crystalline substance is dispersed as the domain (the island part) in the sea part of the amorphous polymer as the main component. However, in order to form this domain structure, it is, of course, impossible to use a crystalline substance as the main component.
In addition, with a toner manufacturing method which uses a crystalline resin to implement the emulsion polymerization aggregation method in an aqueous medium for manufacturing the toner particles, it is extremely difficult to control the particle diameter, the particle size distribution, and the charging characteristic of the toner, which is a major problem in terms of using the product as a toner. This is because it is difficult for the crystalline resin to provide the resin particles to be used for the emulsion polymerization aggregation method in the ordinary aqueous medium with sufficient chemical stability and mechanical stability, and in order to provide these, it is necessary to introduce an ionically dissociatable polarity group into the resin. However, this polarity group generally lowers the crystallinity of the resin, which is a problem in terms of the sharp melt low temperature fixing, at which the present invention aims.
In addition, in this case, in order to provide this function for the amorphous polymere and attach a polarity group to the surface of the particle, the mini-emulsion method as practiced in JP-A 2001-42564, for example, has been tried for copolymerization of a monomer comprising a polarity group with an amorphous polymer followed by use thereof; however, with the mini-emulsion method, mere copolymerization of a polarity group monomer in the aqueous medium is considered to be extremely difficult in terms of the polymerization mechanism, and thus with the emulsion polymerization aggregation method, it is extremely difficult to provide a sufficient control of the toner particle diameter and distribution.
Further, the same problem that the charging characteristic of a toner is poor due to the low volume receptivity of the crystalline resin is present also in a toner formed by the emulsion polymerization aggregation method, and at present, it is difficult to secure the charging characteristic required for electrophotography.
Thus, with the toner produced by the above-mentioned respective methods using the crystalline polymer, it is extremely difficult to secure a sufficient charging characteristic and offset resistance together with the low temperature fixability, which is an advantage of the crystalline polymer, and to provide a sufficient charging characteristic, particle size distribution characteristics, and the like, for the toner, and at present, no toner which meets the requirements for all these toner characteristics has been provided.
By using a polycondensed resin having a polyester structure, or the like, as the toner resin to be applied to the low temperature fixing technique technological development for such a purpose as reducing the energy for fixing, which consumes the highest amount of power in the electrophotographic process, is being actively promoted. This polyester resin has been generally manufactured by a polycondensation reaction at a high temperature above 150° C., but the technology for polymerization and manufacturing with a lower amount of energy (at a lower temperature) has been intensely demanded from the viewpoint of environmental load reduction. As a technique for polymerizing a polyester resin at a lower temperature, it has been found in recent years that polymerization can be carried out at a temperature below 100° C. by using a polymerization catalyst comprising a rare-earth element, such as scandium (Macromolecules, 2003, vol. 36, pp. 1772 to 1774).
However, regarding the polyester resins polymerized with the use of these new polymerization catalysts, their catalytic chemistry, mechanisms, side reactions, residual catalyst effects, and the like are being vigorously investigated at present. However, the investigation of these is still at the stage of basic research, and how they can be industrially put to practical use, what differences they have, as compared to the resins obtained by the conventional manufacturing method, and what characteristics must be controlled to allow them to be put into practical use are important questions in the industry.
On the other hand, with the current rapid popularization of digitalization technology, the demand of users in general houses, offices, and the publishing region for higher image quality in the outputs, such as prints, copies, and the like, is being increased day by day. Then, in response to the demand for higher image quality, improving the resolution by decreasing the particle diameter, particularly for a toner for use in electrophotography, has been recognized as a technically important approach, and at present, it is aimed that the particle diameter for the toner be reduced down to the order of 5 μm. In this case, using the kneading pulverizing method, which has been conventionally used for toner manufacturing, it is difficult to realize a particle diameter as small as less than 6 μm with the particle size distribution being adequately controlled to maintain the desired characteristics of the electrophotography toner, from the viewpoint of manufacturing energy and cost, and at present, the toner manufacturing method is being shifted to the so-called chemical manufacturing method, such as the suspension polymerization method, the dissolution suspension method, the emulsion polymerization aggregation method, and the like, which are performed in an aqueous medium. Therefore, it is more preferable that the low-temperature fixing resin and the low-temperature polycondensation technology which incorporate the low energy and low environmental load technology as described above be applicable in the aqueous medium, however, with the above-mentioned technology which uses the polycondensation mechanism as the fundamental principle, it has been considered to be difficult to implement direct polymerization in the aqueous medium.
Therefore, when such a resin is to be applied to the toner by the chemical manufacturing method, the technique which carries out polycondensation in advance by the bulk polymerization method, the solution polymerization method, or the like to provide a higher molecular weight, and then dispersion-emulsifies the product into the aqueous medium has generally been taken. In this case, in order to provide an adequate performance for a high-image quality toner, it is required to highly control the particle diameter and the distribution as previously described; however, it is extremely difficult to dispersion-emulsify the resin which has once been provided with an increased molecular weight by bulk polymerization or the like, and the dispersion-emulsification would require use of an organic solvent, or the like, high-temperature heating coalescenc, high shearing energy dispersion, or a classification operation at the final process requiring a large amount of energy. Therefore, it is difficult to make decreasing the energy in the electrophotographic process by using a low temperature fixing resin compatible with the low-energy manufacturing technology for the toner resin indispensable for electrophotographic process power reduction, and even if the amount of energy for the electrophotographic process could be reduced by low temperature fixing, or the like, manufacturing of the resin therefor requires much energy, compared to the conventional resin manufacturing. Thus, when the economics of the total energy required at the stages of material manufacture to the use of the product is considered, it cannot be said that energy reduction has been achieved.
In other words, as regards future challenges for toner manufacture, in order to render low environmental load and low energy technologies in electrophotography, such as low temperature fixing compatible with a toner manufacturing method using a chemical method which is indispensable for responding to the demands of recent years for provision of higher image quality, and the like, in electrophotography, and to achieve ideal compatibility, development of technology which can easily manufacture an aqueous dispersion (dispersion) element of the above-mentioned polycondensed resin is considered to be essential. As a remarkable finding for solving this problem, a literature which reports that polycondensation for a polyester in an aqueous medium, which has been considered to be difficult, is possible can be mentioned (Saam J C, Chou Y J. U.S. Pat. No. 4,355,154; 1982).
However, that technology has a number of ambiguous points in the polymerization mechanism, and with only the technology as disclosed in that literature, it is difficult to obtain a high-molecular weight polymer which meets the requirements for the characteristics of the electrophotographic toner, and to assure the high-image quality characteristics such as the particle size distribution, the charging characteristic, and the like, and thus the technology as disclosed in the same literature is not at a level high enough to be applicable to the sphere of toners.
Therefore, as solutions to the above-mentioned various problems with the low temperature fixing technology, there is the need for a toner for developing electrostatic latent images having an excellent particle size distribution characteristics and charging characteristic, and a manufacturing method therefor; an developer for electrostatic latent images using the toner for developing electrostatic latent images; and an image forming method utilizing the toner for developing electrostatic latent images. In addition, there is the need for a method for manufacturing a dispersion of resin particles to be utilized as a constituent material, and the like, for the toner for developing electrostatic latent images, and more specifically, a dispersion of resin particles with which the polyester surface is satisfactorily coated with a radical polymer, the surface exposure of the polyester being suppressed, while resin particles having a small particle diameter and a sharp particle size distribution can be manufactured, the resin particles being stably emulsified and dispersed.